Modified Bitumen Inspection

What is Modified Bitumen? 

Unlike and sometimes confused with asphalt roll roofing, Modified Bitumen (MB) membranes are tougher and resist tear. Asphalt roll roofing is thinner than MB, easily torn and the application usually involves nails and roof cement. An MB membrane is reinforced with fiberglass, polyester or a combination of the two. 

The asphalt (bitumen) is the waterproofing element, which has been modified (or improved) with Polymers. The membrane can be surfaced with mineral granules or metal foil in a variety of colors or left unsurfaced (smooth). MB is often referred to as rubber or a rubberized roof, but it is important to know it is not the same as an EPDM (rubber) roof.

A variety of Polymer blends is available. The two most common modifiers are: Styrene-butadiene-styrene (SBS) and Atactic polypropylene (APP). 

• SBS polymer gives the asphalt a rubber-like characteristic. Known to have good flexibility in cold temperatures, it’s more sensitive to UV radiation than APP. Usually, it’s applied with hot asphalt: however, it can be applied using a cold adhesive. To prevent premature deterioration from UV radiation, granules similar to those found on shingles or metal foils are used to surface this type of membrane.

•APP polymer is known to perform better in hot weather applications, and it is more resistant to UV radiation than SBS. It’s usually applied with a torch (heat-welded); however, some manufactures have cold adhesives available. APP is usually an unsurfaced (smooth) material, but there are granular-surfaced versions available. It is generally recommended that unsurfaced membranes be painted with a reflective paint to increase its resistance to UV radiation. It is my experience that this greatly increases the life of the roof.

On homes, the membrane is usually single-ply, although multi-plies can be installed.

Several manufacturers have introduced self-adhering (peel and stick) versions of MB over the past 10 years. This article does not address this type, but it is important to know they are out there. They have the same basic installation details and defects as other types. I have seen peel-and-stick versions used to repair built-in gutters, as well as entire roofs. From experience, I believe their ability to adhere depends on the condition of the surface, including how well it was cleaned. Like tape, self-adhering MB will not stick to dirt, rust, etc.

THE INSPECTION

Where to start

The goal of a roof inspection is to identify the following:

• roofing materials,
• deficiencies,
• roof components that are not
• functioning properly, and
• components that are near the end of their expected service life.

While on the ground, carefully view any perimeter edge flashing, noting any irregular appearance that deserves a closer evaluation once on the roof. 

On a masonry home, note any efflorescence along the top of exterior walls, which could be related to a roof leak, or any masonry deterioration that could contribute to a roof leak. 

Prior to getting on the roof, thoroughly inspect interior ceilings, walls and attics for evidence of leakage and/or damage. Signs of a leak on a ceiling don’t mean the roof is leaking directly overhead. Often, flat roofs have layers such as base sheets, more than one ply, old roof, rigid insulation or metal decking. The leak may originate in one spot and zigzag down until it finds the path of least resistance before it shows up, often many feet away. On a metal deck, for example, it is possible that water entered on the front high side of the home and trailed down the metal to the rear low side before it found its way down to stain a ceiling. Experienced roofers hired to locate and repair leaks always determine the type of roof decking, asking whether the leak showed during the rain or sometime after. This information helps them find the leak.

What about Structure?

Structural defects can affect roof performance. 

Residential additions with inadequate flat roofs. At times, the once-flat roof has deflected downward several inches into the shape of a bowl, causing cracks and noticeable bowing in the interior ceiling. If it’s a drop ceiling, you can see the bottom side of the structure by removing the ceiling tiles. The decking should not be rusted (if metal); even slight rust should be reported and further evaluated. Rusty roof decking can be an indication of high humidity in the building; but more often indicates faulting roofing. What appears to be surface rust on the underside of metal decking can actually be significant damage, so be careful walking on a rusty metal deck. 

As we focus on MB, please be aware there are many structural issues and roof deck materials not covered in this article. 

Why walk the roof?

Opinions differ on whether to walk on roofs, but I don’t know how anyone can adequately inspect a flat roof without walking on it. When inspecting any roof, always err on the side of safety, always know where you are on the roof and never walk backwards. As you walk around inspecting the roof, pay close attention to how it feels underfoot. Often, rigid insulation is found with MB roofs, although it is less common on homes than in commercial buildings. It is impossible to visually detect wet insulation, and it’s difficult to detect it from feel. A sunken or spongy-feeling area may indicate the insulation is wet and damaged. Sometimes, you can hear the sound of water squishing, or you may see water squirt out from the membrane when pressure is applied. Be careful not to step on blisters or anything else that could damage the roof. 

What are the components of the roof system?

Home inspectors, probably more than anyone including roofers, understand that the home is a system of components where one component can directly affect another. The roof system is no different. Roof system components include the following:

• roof covering (or membrane),
• flashings,
• penetrations,
• drainage,
• ventilation,
• insulation (on a flat roof), and
• deck (or substrate).

If one component fails, it adversely affects the entire roof system. 

The waterproof system of any flat roof has three main parts: 

1. the membrane (or the cap sheet),
2. the seals (or the sealants that form a waterproof joint between two waterproof materials), and
3. the flashing.

The majority of roof failures (leaks) occur at laps, flashings and seals, a small part of the total roof. So, pay careful attention should be given to where the membranes lap, or meet a vertical angle, such as around a skylight, plumbing vent stack, mechanical equipment curb, parapet wall, house exterior wall, or terminate at the perimeter edge or at an installed guard rail. 

What are common membrane issues? 

1) Laps are critical on flat roofs and should be examined for full and uniform adhesion. On an MB roof, you should see the following:

• a uniform flow-out of asphalt at the laps (ideally 3⁄8" of asphalt flow-out at all laps, including base flashing);
• the bottom edge overlapping the adjacent row (a minimum 3"); 
• End laps (or T joints) staggered so no adjacent end laps coincide(a minimum 18") and the end lap overlapping the other end by approximately 6"; and
• Laps parallel to the slope of the roof so the gravity flow of water is never running against the lap.

Examine the laps for indications of short laps, overheating, voids and/or seal deficiencies.

2) Blisters are commonly caused by moisture entrapment within the roof system, but may also be due to poor adhesion, air entrapment and/or vapor. Blisters should always be reported as a deficiency, but the cause and/or the decision whether it should be repaired is best left to a roofing contractor. If not moisture-related, not in a lap and not within a walking path, small blisters are probably insignificant and commonly ignored by roofing contractors.

3) Splits can be an indication of stress or movement within the structure, and, if so, flashing will usually have also failed. In large buildings, stress is controlled with the installation of expansion joints, which are seldom used in homes. Splits are caused by a number of reasons in addition to stress, including faulty installation.

4) Wrinkles (or ridges) in the membrane can be due to movement in the membrane, but also can be an installation flaw. Wrinkles usually end up splitting.

5) Movement, slippage or migration of the membrane is not commonly seen on homes unless the slope is steeper than normal. Usually, it’s related to thermal forces, the lack of adhesion and/or the use of a membrane not rated for a steep slope. Movement of the membrane can contribute to splits, flashing failures and wrinkles.

6) Damage. As with any roof, MB is susceptible to damage. The causes of damage are numerous such as punctures from fallen tree limbs, foot traffic, dropped tools, sharp objects, vibrating roofing installation mechanical equipment, fasteners backing out of substrate puncturing the membrane, etc. Also, rooftop mechanical equipment such as kitchen exhaust fans can discharge cooking oils, etc., which can quickly degrade a roof membrane. Any potential discharge must be controlled to prevent roof damage.